feature-sliced-design

Feature-Sliced Design (FSD) v2.1

Source: fsd.how | Strictness can be adjusted based on project scale and team context.

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1. Core Philosophy & Layer Overview

FSD v2.1 core principle: "Start simple, extract when needed."

Place code in pages/ first. Duplication across pages is acceptable and does not automatically require extraction to a lower layer. Extract only when the same code is currently being used in multiple places (not hypothetically), the usages do not always change together, and the boundary has a focused responsibility.

Not all layers are required. Most projects can start with only shared/, pages/, and app/. Add widgets/, features/, entities/ only when they provide clear value. Do not create empty layer folders "just in case."

FSD uses 6 standardized layers, listed here from highest to lowest:

app/       → App initialization, providers, routing
pages/     → Route-level composition, owns its own logic
widgets/   → Large composite UI blocks reused across multiple pages
features/  → Reusable user interactions (only when used in 2+ places)
entities/  → Reusable business domain models (only when used in 2+ places)
shared/    → Infrastructure with no business logic (UI kit, utils, API client)

Import rule: A module may only import from layers strictly below it. Cross-imports between slices on the same layer are forbidden.

// ✅ Allowed
import { Button } from "@/shared/ui/Button"; // features → shared
import { useUser } from "@/entities/user"; // pages → entities

// ❌ Violation
import { loginUser } from "@/features/auth"; // entities → features
import { likePost } from "@/features/like-post"; // features → features

Note: The processes/ layer is deprecated in v2.1. For migration details, read references/migration-guide.md.

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2. Decision Framework

When writing new code, follow this tree:

Step 1: Where is this code used?

• Used in only one page → keep it in that pages/ slice.
• Used in 2+ pages but duplication is manageable → keeping separate copies in each page is also valid.
• An entity or feature used in only one page → keep it in that page (Steiger: insignificant-slice).